bookmark: simplify nodemap check introduced in the previous changeset stable

bookmark: simplify nodemap check introduced in the previous changeset

--- a/mercurial/bookmarks.py	Thu Oct 25 23:43:52 2012 +0200
+++ b/mercurial/bookmarks.py	Fri Oct 26 09:29:50 2012 +0200
@@ -268,7 +268,7 @@
                     # obsolescence marker does not apply to public changeset
                     succs.update(obsolete.allsuccessors(repo.obsstore,
                                                         [c.node()]))
-            known = (n for n in succs if nm.get(n) is not None)
+            known = (n for n in succs if n in nm)
             validdests = set(repo.set('%ln::', known))
         validdests.remove(old)
         return new in validdests